Patent GB-727770-A (GB)
Title: Improvements in digestion of phosphate rock
IPC: C05B11/06
Filed: 1953-04-20
Abstract: A non-hygroscopic tri-calcium phosphate is made by dissolving the phosphatic component of phosphate rock in an inorganic nitrogenous acid, incorporating potassium chloride, sodium chloride or sodium sulphate into the so formed aqueous phase in amount at least stoichiometrically equivalent to the calcium nitrate content, as well as sufficient to precipitate a major portion of any acid-soluble fluorine component of the rock as fluosilicate, and neutralizing the aqueous phase with ammonia, whereby the substantially nonhygroscopic tri-calcium phosphate is precipitated.  Nitric acid is preferably used for the digestion, but the solution produced, if desired in situ, by passing gaseous nitrogen oxides through water or other aqueous liquor is mentioned.  It is preferred, when alkali metal salts are added in solid form to heat the aqueous phase to assist solution.  The ammonia for neutralization is preferably used in anhydrous form, though aqueous ammonia may be used; the quantity is preferably such as finally to provide a slightly alkaline pH. Separation of solids from the mixture may, if desired, be performed after digestion to remove the insoluble portion of the phosphate rock, after adding the alkali metal salt to remove fluosilicate and/or after precipitation of the tri-calcium phosphate, but if the product is to be used solely as fertilizer these insoluble portions may be allowed to remain.  The liquid obtained after removal of tri-calcium phosphate may be concentrated and the chloride or sulphate of ammonia and nitrate of sodium or potassium crystallized and removed together, or separately; these salts may, if desired, be added to the calcium phosphate to produce a fertilizer.  Recycling to the digestion or any succeeding step, of the aqueous solution left after removal of the precipitate of tri-calcium phosphate (and possibly other salts), if desired after concentrating, is referred to, but if a sodium salt is added, the liquor should not be added before precipitation of fluosilicate. In the example phosphate rock ground to 80 per cent. minus 100 mesh (Tyler) was treated with a 35 per cent. nitric acid solution. The temperature rose to 40 DEG  C. and the mixture was agitated until the cessation of gas evolution indicated the completion of reaction.  The solids were removed by filtration and reintroduced with fresh rock in a subsequent extraction.  A saturated solution of potassium chloride at 100 DEG C. were added, and the mixture agitated; potassium fluosilicate was removed by filtration, and anhydrous ammonia added to a pH of 7.  The precipitate of phosphate was filtered off and dried without washing at 100 DEG  C.: analysis then gave 20 per cent. total P2O5, 19.5 per cent. available P2O5, 5 per cent. N, 8 per cent. K2O and about 2 per cent. water.  More moisture was removed at a higher temperature.  The filtrate was then concentrated by evaporation and the potassium nitrate and ammonium chloride which separated, added to the tri-calcium phosphate, whilst the mother liquor which contained excess potassium chloride was used as the medium in which fresh potassium chloride was added to the process.ALSO:A non-hygroscopic, non-caking and free flowing tri-calcium phosphate, suitable as a livestock feed supplement or fertilizer and stable under storage conditions, is made by dissolving the phosphatic component of phosphate rock in an inorganic nitrogenous acid, incorporating potassium chloride, sodium chloride or sodium sulphate into the so formed aqueous phase in amount at least stoichiometrically equivalent to the calcium nitrate content, as well as sufficient to precipitate a major portion of any acid-soluble fluorine component of the rock as fluosilicate, and neutralizing the aqueous phase with ammonia, whereby the substantially non-hygroscopic tri-calcium phosphate is precipitated.  Separation of solids from the mixture may if desired be performed after digestion to remove the insoluble portion of the phosphate rock, after adding the alkali metal salt to remove fluosilicate and/or after precipitation of the tri-calcium phosphate, but if the product is to be used solely as fertilizer these insoluble portions may be allowed to remain.  The liquid obtained after removal of tri-calcium phosphate may be concentrated and the chloride or sulphate of ammonia and nitrate of sodium or potassium added to the calcium phosphate to produce a complete fertilizer.  In the example, phosphate rock ground to 80 per cent. minus 100 mesh (Tyler) was treated with a 35 per cent. nitric acid solution.  The temperature rose to 40 DEG C. and the mixture was agitated until the cessation of gas evolution indicated the completion of reaction.  The solids were removed by filtration and reintroduced with fresh rock in a subsequent extraction.  A saturated solution of potassium chloride at 100 DEG C. were added, and the mixture agitated; potassium fluosilicate was removed by filtration, and anhydrous ammonia added to a pH of 7. The precipitate of phosphate was filtered off and dried without washing at 100 DEG C.: analysis then gave 20 per cent. total P2O5, 19.5 per cent. available P2O5, 5 per cent. N, 8 per cent. K2O and about 20 per cent. water.  More moisture was removed at a higher temperature.  The filtrate was then concentrated by evaporation and the potassium nitrate and ammonium chloride which separated added to the tri-calcium phosphate, whilst the mother liquor which contained excess potassium chloride was used as the medium in which fresh potassium chloride was added to the process.